Genetics

GENETICS (from Greek genesis-origin), is usually defined as the physiology of variability and heredity. It was thus that Bateson defined the content of genetics in 1906, when he proposed this term, wishing to emphasize that of the three main elements of the evolutionary process-variability, heredity, and selection (struggle for existence)-the first two must be studied precisely by the methods of physiological sciences and must also be studied together. However, in the time that has passed, Genetics has included a number of questions of a more general nature than "variability" and "heredity" (for example, the question of sex, of the internal structure of the chromosomal apparatus, of the relationships between genes and traits, etc.), and has closely touched, on the one hand, with the mechanics of development, on the other-with cytology, and, on the third-with biochemistry, so that Bateson's definition of Genetics must already be recognized as too narrow. At present, Genetics is one of the sciences of the structure of organisms insofar as in the organism one can distinguish the structure of 1) anatomical, 2) histological (and cytological), 3) genetic, 4) chemical, and perhaps others. Located between histology, which works by the optical method, and chemistry, which works by the method of chemical analysis and synthesis, Genetics deals with elements that are usually already inaccessible to optical study and still inaccessible to chemical study. These structural elements, called genes (see.), become accessible to study due to the fact that, being in the organism, they influence the development of its traits, or phenes, and observation of these traits opens the possibility of making conclusions about the properties of the genes themselves. But just as chemistry can say almost nothing about the structure of chemical compounds until they react with other chemical compounds, so Genetics learns about the genetic structure of the organism only by observing its changes, i.e., by studying variability (see.), or by crossing it and studying heredity (see.), for which there exists a whole system of methods called genetic analysis (see.). Thus, the study of variability and heredity, having enormous significance as an end in itself, is at the same time the main method of studying the genetic structure of organisms. In recent times, genetics has also tried to further expand its boundaries, setting alongside questions about the genetics of the organism the question of G. of various aggregates of organisms, so-called populations (see.)-races, species, and others, since at this scale it has been possible to discover regularities that require further study. But since the structure of modern populations cannot be fully understood without knowledge of their history, the study of population dynamics must also enter into the circle of genetics. Here the problems of genetics approach closely to the problems of evolution. G. is divided into general and private. General G. investigates the regularities that extend to more or less extensive groups of organisms and the concrete manifestations of these regularities in individual cases. Private G., on the contrary, places the given type of organism at the center of its attention. If, for example, general G. establishes that "genes are arranged in chromosomes in linear order," then private G. of poultry clarifies which specific genes and how, in which chromosome, are located poultry genes, what significance they have, as well as how they can be used in a practical sense (applied genetics). History of G. Since G. developed on the soil of the study of heredity and variability, the roots of its history go back to the centuries-old experience of livestock and crop breeders, especially-in countries of industrial pedigree livestock breeding and crop cultivation. Constantly dealing with heredity as a fact, they outlined rules and theories, often fantastic, while at the same time accumulating facts and emphasizing the importance of the problem. Due to the lower cost of material and simplicity of work, botanists preceded zoologists, and among the first there are a number of names of researchers-Koelreuter (1761), Knight (1779), Gärtner (1849), Naudin (1862), who, persistently performing crossings and outlining regularities, came very close to the discovery of the laws of inheritance, which was achieved, however, only by the genius of Mendel (1865; see Mendelism). Mendel's discovery, however, turned out to be premature, as biology had not yet had time to prepare the soil for its broad generalization. This soil had to be prepared by cytology by studying the detailed structure of the cell, by the discovery of the nucleus, by studying the chromosomal apparatus and reduction division,-which was accomplished by it approximately by 1900 on the soil of the corresponding progress of optical and chemical technology. By this time, the development of the evolutionary idea, the dispute between neo-Darwinism and neo-Lamarckism had sharpened the attention of biologists to the significance of the problem of "germ plasm," the theory of which had been developed to a large extent speculatively, starting from 1883, by A. Weismann. Finally, the last root of genetics lies in the works of Galton and then Pearson, in a series of studies that developed the mathematical method in the study of variability and partly of heredity under the name of biometry (see.). By 1900, the soil for broad genetic generalizations, thus, had ripened. De Vries, on the basis of his many years of work, put forward the "mutation theory," one of the most important points of which was the doctrine of individual traits, almost alien to previous biologists, but coinciding with Mendel's concepts. The simultaneous publication of a number of other works revived Mendel's name and confirmed the correctness of his discoveries. In 1903, Johannsen, with his classic research on "pure lines," substantiated the second most important point of G.-the difference between "genotype" and "phenotype." Mendelism in these years and the following quickly won positions through the work of numerous researchers, both botanists and zoologists (Correns, Baur, Punnett, Bateson, Davenport, Castle and others), of whom Bateson and Punnett created the theory of "presence and absence" to explain the phenomena of Mendelism. In 1910, Nilsson-Ehle, with his theory of "multiple genes," made possible the Mendelian interpretation of so-called intermediate heredity, thus significantly expanding the boundaries of Mendelism, which claimed rights to the significance of a universal theory of heredity, and not only one of its types. At the same time, biometry also had the opportunity to merge closely with genetics, since the doctrine of distribution curves, etc., could be applied to multiple genes. Somewhat earlier, in 1902, McClung gave an explanation of "sex determination" by means of sex chromosomes (see.), and soon after-first by Doncaster (1906), and then Pearl and others.-sex-linked inheritance was discovered, which included in G. the problem of sex and indicated the connection of Mendelian heredity phenomena with chromosomes. From 1910, the work of Morgan's laboratory began, brilliantly developing the chromosomal theory of heredity, which is the most important part of modern G. The last historical date in the field of G. is the discovery in 1927 by Müller of the method of artificially obtaining mutations (see.) by means of X-rays, which will undoubtedly further advance the development of this science. The current state of G. amounts to the following. In any organism, G. first of all distinguishes its phenotype (see.), or the aggregate of traits, phenes, from its genotype (see.), or the aggregate of predispositions, genes (see.). The genotype of an organism is determined either at the moment of egg formation (in parthenogenesis), or at the moment of fertilization, or in asexual reproduction, at the moment of separation of a bud, petiole, etc. The subsequent development of this embryo then begins through increase, complication, and differentiation, leading to the emergence of many new qualities, traits of the organism, both externally noticeable (color, size, forms), and invisible (chemical peculiarities of body fluids, physiology of functions, instincts, mental abilities, etc.), which constitute its phenotype. With different genotypes, the phenotype is usually also different, but the phenotype is not entirely determined by the genotype, since the development of traits also depends on external conditions. In the study of traits of various organisms, regularities are discovered, for example, many mammals have pied forms, in which in the distribution of patches there are repeating regularities, apparently connected with the mechanics of development, the study of which is occupied by phenogenetics. Due to the dependence of the phenotype on external conditions, organisms, even having completely identical genotypes, i.e., belonging to one biotype (see.), turn out to be phenotypically different, forming fluctuating modifications (see Fluctuations, Modifications, Variability). The study of this fluctuating variability reveals its subjection to regularities that can be mathematically formulated (see Biometry, Variability), as well as the fact that even strong changes in the phenotype are not accompanied by adequate changes in the genotype. Only in organisms in which the embryo is closely connected with the maternal organism (for example, in mammals), and under the special connections of generations in protozoa, modifications of one generation have the opportunity to be reflected in the next, forming "long-lasting modifications," gradually fading away.

Finally, with modifications of one trait, other traits are also usually changed, and correlations are often found between these changes, studied by biometrics. In recent times, R. Goldschmidt has introduced many interesting ideas into the question of trait development. The genotype of an organism consists of genes, which are most closely associated with chromosomes (see) and which are apparently parts of them. Together with the chromosomes, genes are present in all cells of the body, so that, despite the negligible size of each gene individually, in total they constitute a noticeable part of the organism. The theory of 'linear arrangement of genes', based on the study of the phenomenon of crossing-over or crossing, proves that genes are arranged in chromosomes like beads strung on a thread, or like amino acids in the chain molecule of polypeptides. However, if the diameter of a gene turns out to be smaller than the diameter of the chromosome, then it becomes possible for several genes to be located at one point along the length of the chromosome but on different sides of the chromosome, and clarification of this will require a different method. A change in any gene usually reflects in the phenotype a change in any trait, which is why the terminology arose: 'color gene', 'gene for leg elongation', 'enhancer gene for milk production', etc. These names are conventional, since in different cases the same gene can manifest differently and, in addition to the most noticeable manifestation, has others of varying degrees of noticeability. But with these reservations, parallel with 'individual genes', they speak of corresponding 'individual traits'. Consideration of genes located one after another in any chromosome reveals so far a complete absence of connection between the location of genes and the nature of traits; adjacent genes can influence completely different traits, while genes located far apart or even in different chromosomes can influence similar traits. It is possible, however, that this impression is temporary, and when a much larger number of genes in each chromosome is studied, the sought-after regularities will be found, hints of which already exist. Physically, therefore, genes are collected into chromosomes, and chromosomes into a set of chromosomes, an idiogram, or the 'chromosomal apparatus'. If genes are invisible, then chromosomes are already accessible to microscopy, and their study constitutes a significant part of karyology (see). It reveals that each species of organisms is characterized by a characteristic idiogram, i.e., the number, size, shape and appearance of chromosomes. In each cell of the body, the number of chromosomes is twice as large as in the gametes (eggs and spermatozoa). The smaller number is called haploid, the larger one diploid. In the diploid set, each chromosome turns out to be paired (except for part of the sex chromosomes). Consequently, the set of genes in gametes is haploid, and in body cells is diploid. In recent times, cases are increasingly found where in an organism or part of it the set of chromosomes, and therefore of genes, turns out to be triple, quadruple, and up to tenfold, and from this duality, triplicity... decimality of the organism, many peculiar properties, varying degree and ability to changeability, different types of heredity, etc., follow. It is not uncommon for the entire set of chromosomes to be tripled or quadrupled, but only part of them, so that part of the genes is also multiplied. Such organisms arise either from crossings or under the influence of external influences, and now the methodology for their experimental production is being perfected more and more (see Mutations). Together with the chromosomes, genes pass from one generation to the next: a diploid organism forms haploid gametes, when two gametes meet, a new diploid organism arises (in the case of polyploidy this phenomenon is complicated). The process of gamete formation and their union constitutes the content of heredity, the study of which is the main source of our genetic knowledge (see Mendelism, Heredity). Optical study of chromosomes gives only the most superficial acquaintance with them; the method of genetic analysis, however, allows one to penetrate into the structure of chromosomes much more deeply. By this method, plans of chromosomes are made, indicating the location of genes on them. When considering such a plan, it is found that the genes on the plan are arranged unevenly (in well-studied species of Drosophila), forming clusters in some places, thinning out in others. In the sense of the method of making the plan, these thickenings apparently indicate different mechanical properties of chromosomes in different areas. Before the formation of gametes, chromosomes of the same pair (homologous chromosomes) cross over, or crossing-over, exchanging with each other homologous regions. For such an exchange, they must obviously undergo a break in continuity. The study of the location of these break points along the length of the chromosomes, the mutual location of two, three points reveals the peculiar 'elastic properties' of chromosomes (see Interference), reveals the existence of 'special points', etc. Changes in external conditions, temperature, age of individuals, the action of X-rays, etc., reveal the different reaction of different parts of chromosomes to these influences, proceeding along characteristic curves. In this direction, the study of the internal structure of chromosomes still has a great deal to do, and here genetics will probably closely approach chemistry: even now voices are heard that in the face of chromosomes we are dealing, perhaps, with giant molecules. If physically genes are grouped into chromosomes, then physiologically they can be grouped into genomes. One genome is formed by genes (one of each), the minimum number of which is necessary for the formation of a viable individual possessing all the traits of a given species of animal or plant. Usually an organism has at least two genomes, since even the haploid set of chromosomes carries the entire set of genes that make up the genome. However, even the heterogametic sex (males of mammals, females of birds, etc.), having an XY structure, does not have two complete genomes, since the Y chromosome is unpaired, and the haploid set with a Y chromosome is incapable of producing a viable organism. The concepts of genome and haploid set do not coincide also because in the latter it is possible to assume the existence of some genes in doubled amount due to duplications, etc. The study of hybrids of some plants, for example, poppies, has made it probable that in them normally in the haploid set there is more than one, perhaps even several genomes. The presence of several genomes Winge tries to explain the rarity of mutations in the autosomes of the fish Lebistes, in which, on the contrary, mutations in the sex chromosomes are very frequent. In this area there is still much unexplored. In recent times, by means of interspecific hybridization, it has been possible to obtain plants in each cell of which there are complete sets of genes of two species (e.g., radish and cabbage in the experiments of Karpechenko). Since part of the genes in radish and cabbage are apparently the same, and part different, in hybrids some genes turn out to be quadrupled, others doubled, and the counting of genomes is very difficult. Meanwhile, for evolutionary theory the concept of genomes is very important. The genetic structure of some organisms (mainly plants), called 'chimeras', turns out to be very peculiar. The outer layers (or layer) of cells of chimeras have a different genotype than the inner layers. Sometimes this is a difference in only one gene (some varieties of potato), but sometimes different layers belong to different species and even genera (Crategomespilus). The phenotype of such chimeras is a peculiar resultant of two different genotypes (apart from external conditions). At present, experimental methods for obtaining chimeras (Winkler) and 'dechimerization' (Aseeva) have been found, and the study of these phenomena should give much for understanding the connections between phenotype and genotype, the action of genes, etc. Passing from the genetics of the individual to the genetics of the species, we see first of all the brilliant results of genetic analysis of polymorphism, dimorphism, and in particular sexual dimorphism (see Sex). Dioecious species of animals and plants turned out in most cases to be a collection of two genotypes, connected, however, into a single system due to the fact that none of these genotypes is capable of reproducing individually. One genotype is characteristic of females, the other of males of the species. In many cases, the difference between genotypes can be seen under the microscope, due to the difference in the number of chromosomes or their external appearance. Usually in such cases one genotype turns out to be homozygous, and the other heterozygous for all those genes that are localized in the sex chromosome, and the reproduction of such species even in pure lines amounts essentially to constant crossing of a heterozygote with a homozygous recessive (XX), which in the offspring again gives 50% XY-heterozygotes and 50% XX-homozygotes, i.e., to two sexes in the ratio 1:1. A similar scheme also explains the structure of species having several types of females (e.g., in the genus Papilio). The study of the genetics of sex has proved very fruitful for clarifying the nature of gene action.

In Drosophila, for example, besides the two normal sexual forms, male and female, it has also been possible to obtain several others—'supermale', 'superfemale', 'intersexes', and the difference between them proved to be connected with the proportion of X chromosomes in relation to the others. The aggregate of genes localized in the X chromosome shifts the development of the organism in the 'female direction', and the greater the proportion of X chromosomes in the set of chromosomes, the more sharply is 'femininity' expressed (for more details see Sex). In each chromosome, apparently, there are genes acting both in one and in the opposite direction, which is why the chromosome as a whole has a certain 'balance' depending on whether 'male' or 'female' genes predominate. The summation of the balances of individual chromosomes comprising the set results in some general total determining the direction in which the development of the organism will proceed. Since changes in the set of chromosomes also change other characteristics besides sex, the same reasoning applies to them as well. These important conclusions make it possible to interpret the paradoxical fact that a sharp change in the number of chromosomes is often accompanied by almost no change in characteristics, whereas a change in any point of the chromosome leads to a sharp change in the characteristic(s). The changes in characteristics are especially indistinct when there is a multiple increase in the number of chromosomes—the equilibrium of genes remains unchanged, and the small changes in phenotype can to a large extent be explained by the fact that polyploidy is accompanied by a general increase in cell size. Here opens up a very little studied area—the connection between the action of genes and protoplasm. Cases similar to those observed in bees are particularly important here, where males differ from females only by haploidy, and to explain the difference it is necessary to take into account not only the irritant (genes) but also the irritated (protoplasm). Indications of the genetic role of protoplasm are given by some other cases, for example, the experiments of Sturtevant on interspecific hybridization in Drosophila: females obtained from two opposite crossings of two species have an identical genotype, but in one case they are viable, in the other they are not. The explanation of these cases from the point of view of the role of protoplasm is quite difficult. How far does the action of a gene extend? This question is answered by the study of chimeras, mosaics, gynandromorphs, etc. In these cases it is seen that neighboring cells of the organism can have different genotypes, whether due to the peculiar origin of different layers of cells (chimeras) or due to the uneven distribution of chromosomes or genes among the cells of the body. Accordingly (for example, in insects, plants), different parts of the body exhibit the characteristics (phenes) of those genes that are present in the cells of that part of the body. The boundaries of characteristics are often so sharply defined that one has to recognize the narrowly localized action of genes, perhaps not extending beyond the limits of the cell. In some cases, for example, in mammals, this locality is obscured by the fact that a gene can act locally on the function of an endocrine gland, and this gland in turn affects the whole body or its various distant parts. Then removal of the gland creates the impression of removal of the gene, as, for example, in the case of castration of female chickens, which of course is incorrect. G. of aggregates. The problem of G. can be extended, and instead of the question of the genetic structure of an individual, one can pose the question of the genetic structure of a whole aggregate of individuals connected by some unity (species, geographical group of individuals, class of human society, etc.). The most interesting aggregates are 'populations' connected by free cross-fertilization. In this case, the G. of a population acquires remarkable features. The formulas of Mendelism take on a more general character, the change in genotype by mutations becomes gradual with respect to the whole population. Unfortunately, this area of genetics is still little developed (see Population). The private G. of each species of organisms (and anthropogenetics in particular) should include: 1) Analytical G., studying the properties of individual genes and their various combinations, the number of which is determined by the formula 2^n, where n is the number of genes. 2) Topographical G., studying the location of genes on chromosomes and the properties of each chromosome. 3) Geographical G., studying the geographical distribution and distribution of genes and the processes occurring in geographical populations. 4) Historical G., considering the history of genes and combinations. 5) Comparative G., comparing the G. of neighboring species, genera, etc., and 6) Applied G., merging with selection (see) in animal and plant breeding and with eugenics (see) in humans, solving questions of the practical use of G. of a given species of organisms. A sufficiently complete private G. does not yet exist for any organism. Even for such well-studied forms as Drosophila, corn, chicken, we are still very far from sufficiently complete knowledge even of analytical G., not to mention the rest. Domestic animals and plants and very few wild ones are being systematically studied: Drosophila, the silkworm, the wasp Habrobracon, the aquarium fish Lebistes; from plants: Datura, Viola, Crepis, Oenothera sp. sp. The study of private G. will have to give a completely new basis for evolutionary constructions, but in view of the enormous volume of this work, it progresses very slowly. Applied G. relies mainly on the elucidation of the laws of heredity, in particular—on Mendelism, on the doctrine of genotype and phenotype, on the doctrine of pure lines, of lethal genes (see) etc. The discovery of the possibility of creating all possible combinations of genes from the existing set of genes at will gave G. a creative, active character, distinguishing it from all other sciences about the structure of organisms, which was appreciated by practice from the very first steps. On the other hand, genetics allowed animal and plant breeders to provide a theoretical basis for a number of methods that had been empirically discovered by them earlier (inbreeding, the selection method of Svalöf), and to explain a number of phenomena that had remained incomprehensible (heterosis, sex determination, atavism, new formations in crosses, etc.). As for the use in practice of private G., it is still premature to speak of it in view of the extremely insufficient study of it for the most important domestic animals. As for plant breeders, they already base their work entirely on G., and 'pure lines', 'Mendelism', etc. are the basis of the breeding work of a number of scientific and applied botanical institutions both in the West and in the USSR. For medicine, both the general principles established by G. and the private G. of man (anthropogenetics) are of great importance. The doctrine of genotype and phenotype, of hereditary and non-hereditary characteristics, of Mendelian segregation, of mutations, lethal genes, of the peculiar course of heredity linked to sex, etc., allows a physician to understand many pathological phenomena to a greater extent than was possible before, and to more correctly distinguish in etiology the social moment from the biological one. For example, the question of the so-called 'degeneration' for a physician oriented in G. now appears in a completely different light and can be broken down by him into many phenomena of deeply different nature (phenotypic changes, long-term modifications, disharmonious combinations, lethal genes, etc.).—Private G. of man, since it is still unsatisfactorily studied, has for now less importance, but continuously increasing. For example, the realization that hemophilia is a recessive mutation in the sex chromosome allows it to be much more clearly separated from similar but differently inherited diseases (for example, Werlhoff's disease) and contributes to the study of both. The fact that many diseases are inherited differently in different families indicates that in these cases different diseases are denoted by one name, which it will still be necessary to learn to distinguish.—For eugenics, striving to improve the 'human breed', the detailed significance of G. is obviously just as necessary as for the livestock breeder, with the difference that for carrying out this work in humanity it is necessary to elucidate such processes, the control of which will be possible under the given social conditions.—Finally, it is necessary to note the significance of genetics for forensic medicine, in particular, for example, in deciding the question of paternity, which can be given scientifically only on the basis of the most detailed knowledge of anthropogenetics. The study of G. is now conducted in most countries, especially in countries with progressive agriculture (U.S.A., Scandinavia, USSR) and old livestock breeding and seed production (England, Germany). The Romance countries (France, Italy, Spain) hardly participate in this work. In most countries there are genetic societies. International genetic congresses have been held five times, although only the 4th (Paris, 1911, 300 members) and 5th (Berlin, 1927, 1,000 members) deserve this name. From Russia there were 2 representatives at the 4th congress, from the USSR at the 5th—64.

In Germany, the center for the study of G. is Berlin (Kaiser-Wilhelm Institute for Biology headed by Correns and Goldschmidt, Institute for Heredity Research of the Agricultural High School headed by E. Bauer); in Denmark - the laboratories of Winge in Copenhagen; in Sweden - Svalöf (Nilsson-Ehle); in Norway - Oslo (Bonnevie, Mohr); in England: Cambridge (Punnett), Oxford, London (John Innes Horticultural Institute), Edinburgh (Crew). In the U.S.A. G. is experiencing a powerful flourishing. At Columbia University (New York) Morgan, Bridges, and Sturtevant worked (Morgan's laboratory, which has now moved to a new institute in California); at the Station for Experimental Evolution of the Carnegie Institute - Davenport, Blakeslee, Mertz, Demerec, and others. At the Bussey Institute of Harvard University Castle, in Texas - Muller (H. J. Muller), and at many experimental stations and provincial universities a number of other, often very large geneticists: Pearl, Jennings, Emerson, Sewall Wright, and a number of authors of genetic guides who are widely known: Babcock and Clausen (California), Sinnott and Dunn (Storrs), East and Jones, Gowen, Shull, and others. In the USSR G. is developing rapidly. Botanist geneticists are grouped around the Institute of Applied Botany in Leningrad and in Tsarskoye Selo (Vavilov, Karpchenko, Filchenko, Levitsky, and others), the Timiryazev Institute in Moscow (S. G. and M. S. Navashin); they work in Odessa - Sapiegins, and in Saratov - Meister. Zoologists are grouped around the Institute of Experimental Biology in Moscow and its associated Central Genetic Station (Koltsov, Chetverikov, Vasiny, Serebrovsky, and others), the Zoological Garden (M. Zavadovsky, Ilyin, Blyakher), and in Leningrad - in the laboratory of Yu. A. Filchenko (State University).

A. Serebrosky